Receiver sheet for thermal dye transfer printing

ABSTRACT

A thermal transfer printing receiver sheet for use in association with a compatible donor sheet. The receiver sheet has a dye-receptive receiving layer and an opaque biaxially oriented supporting polyester substrate containing (i) small voids, formed around inorganic filler particles, having a mean void size in the range from 0.3 to 3.5 μm, and (ii) large voids, formed around organic filler particles, having a mean void size in the range from 5 to 21 μm and less than 15% by number of the voids have a void size greater than 27 μm.

This invention relates to thermal transfer printing and, in particular,to a thermal transfer printing receiver sheet for use with an associateddonor sheet.

Currently available thermal transfer printing (TTP) techniques generallyinvolve the generation of an image on a receiver sheet by thermaltransfer of an imaging medium from an associated donor sheet. The donorsheet typically comprises a supporting substrate of paper, syntheticpaper or a polymeric film material coated with a transfer layercomprising a sublimable dye incorporated in an ink medium usuallycomprising a wax and/or a polymeric resin binder. The associatedreceiver sheet usually comprises a supporting substrate, of a similarmaterial, preferably having on a surface thereof a dye-receptive,polymeric receiving layer. When an assembly, comprising a donor and areceiver sheet positioned with the respective transfer and receivinglayers in contact, is selectively heated in a patterned area derived,for example from an information signal, such as a television signal, dyeis transferred from the donor sheet to the dye-receptive layer of thereceiver sheet to form therein a monochrome image of the specifiedpattern. By repeating the process with different monochrome dyes,usually cyan, magenta and yellow, a full coloured image is produced onthe receiver sheet. Image production, therefore depends on dye diffusionby thermal transfer.

Although the intense, localised heating required to effect developmentof a sharp image may be applied by various techniques, including laserbeam imaging, a convenient and widely employed technique of thermalprinting involves a thermal print-head, for example, of the dot matrixvariety in which each dot is represented by an independent heatingelement (electronically controlled, if desired).

Available TTP print equipment has been observed to yield defectiveimaged receiver sheets comprising inadequately printed spots ofrelatively low optical density which detract from the appearance andacceptability of the resultant print. There are at least two types ofprinting flaws. The first type are regularly spaced flaws which are dueto gaps appearing between the printed image of adjacent pixels. Theregularly spaced flaws are believed to result from inadequateconformation of the donor sheet to the print head at the time ofprinting. The second type of flaws are smaller and irregularly spacedand are believed to be the result of imperfections in the surface of thereceiver sheet. There is a requirement to eliminate both regularly andirregularly spaced printing flaws, without the need of an additionallayer, and also to provide a very white receiver sheet to enhance thecolours of the printed sheet.

We have now devised a receiver sheet for use in a TTP process whichreduces or substantially eliminates at least one or more of theaforementioned problems.

Accordingly, the present invention provides a thermal transfer printingreceiver sheet for use in association with a compatible donor sheet, thereceiver sheet comprising a dye-receptive receiving layer to receive adye thermally transferred from the donor sheet, and an opaque biaxiallyoriented supporting polyester substrate comprising (i) small voids,formed around inorganic filler particles, having a mean void size in therange from 0.3 to 3.5 μm, and (ii) large voids, formed around organicfiller particles, having a mean void size in the range from 5 to 21 μmand less than 15% by number of the voids have a void size greater than27 μm.

The invention also provides a method of producing a thermal transferprinting receiver sheet for use in association with a compatible donorsheet, which comprises forming an opaque biaxially oriented supportingpolyester substrate comprising (i) small voids, formed around inorganicfiller particles, having a mean void size in the range from 0.3 to 3.5μm, and (ii) large voids, formed around organic filler particles, havinga mean void size in the range from 5 to 21 μm and less than 15% bynumber of the voids have a void size greater than 27 μm, and applying onat least one surface of the substrate, a dye-receptive receiving layerto receive a dye thermally transferred from the donor sheet.

In the context of the invention the following terms are to be understoodas having the meanings hereto assigned:

sheet: includes not only a single, individual sheet, but also acontinuous web or ribbon-like structure capable of being sub-dividedinto a plurality of individual sheets.

compatible: in relation to a donor sheet, indicates that the donor sheetis impregnated with a dyestuff which is capable of migrating, under theinfluence of heat, into, and forming an image in, the receiving layer ofa receiver sheet placed in contact therewith.

opaque: means that the substrate of the receiver sheet is substantiallyimpermeable to visible light.

voided: indicates that the substrate of the receiver sheet preferablycomprises a cellular structure containing at least a proportion ofdiscrete, closed cells.

film: is a self-supporting structure capable of independent existence inthe absence of a supporting base.

The substrate of a receiver sheet according to the invention may beformed from any synthetic, film-forming, polyester material. Suitablematerials include a synthetic linear polyester which may be obtained bycondensing one or more dicarboxylic acids or their lower alkyl (up to 6carbon atoms) diesters, eg terephthalic acid, isophthalic acid, phthalicacid, 2,5-, 2,6- or 2.7-naphthalenedicarboxylic acid, succinic acid,sebacic acid, adipic acid, azelaic acid, 4,4'-diphenyldicarboxylic acid,hexahydro-terephthalic acid or 1,2-bis-p-carboxyphenoxyethane(optionally with a monocarboxylic acid, such as pivalic acid) with oneor more glycols, eg ethylene glycol, 1,3-propanediol, 1,4-butanediol,neopentyl glycol and 1,4-cyclohexanedimethanol. A polyethyleneterephthalate or polyethylene naphthalate film is preferred. Apolyethylene terephthalate film is particularly preferred, especiallysuch a film which has been biaxially oriented by sequential stretchingin two mutually perpendicular directions, typically at a temperature inthe range from 70 to 125° C., and preferably heat set, typically at atemperature in the range from 150 to 250° C., for example as describedin GB-A-838,708.

A film substrate for a receiver sheet according to the invention isbiaxially oriented, preferably by drawing in two mutually perpendiculardirections in the plane of the film to achieve a satisfactorycombination of mechanical and physical properties. Formation of the filmmay be effected by any process known in the art for producing abiaxially oriented polyester film, for example a tubular or flat filmprocess.

In a tubular process simultaneous biaxial orientation may be effected byextruding a thermoplastics polyester tube which is subsequentlyquenched, reheated and then expanded by internal gas pressure to inducetransverse orientation, and withdrawn at a rate which will inducelongitudinal orientation.

In the preferred flat film process a film-forming polyester is extrudedthrough a slot die and rapidly quenched upon a chilled casting drum toensure that the polyester is quenched to the amorphous state.Orientation is then effected by stretching the quenched extrudate at atemperature above the glass transition temperature of the polymer.Sequential orientation may be effected by stretching a flat, quenchedextrudate firstly in one direction, usually the longitudinal direction,ie the forward direction through the film stretching machine, and thenin the transverse direction. Forward stretching of the extrudate isconveniently effected over a set of rotating rolls or between two pairsof nip rolls, transverse stretching then being effected in a stenterapparatus. Stretching is effected to an extent determined by the natureof the film-forming polyester, for example a linear polyester is usuallystretched so that the dimension of the oriented polyester film is from2.5 to 4.5, preferably 3.0 to 4.0 times its original dimension in eachdirection of stretching. The substrate is preferably stretched from 2.8to 3.4, more preferably 3.0 to 3.2 times in the longitudinal direction,and from 3.0 to 3.6, more preferably 3.2 to 3.4 times in the transversedirection.

A stretched film may be, and preferably is, dimensionally stabilised byheat-setting under dimensional restraint at a temperature above theglass transition temperature of the film-forming polyester but below themelting temperature thereof, to induce crystallisation of the polyester.

In order to produce a film having voids, it is necessary to incorporatevoiding agents into the polyester film-forming composition. Voidingoccurs during the film stretching process as a result of separationbetween the polyester and the voiding agent. The size of the voids isdependant upon a complex interaction of factors, such as the chemicalcomposition of the voiding agent and the polyester substrate, theparticle size of the voiding agent, the temperature and shear of theextrusion process, the degree and temperature of the film stretching andpost-stretching crystallisation processes.

By void size is meant the size of the maximum dimension of the void. Theshape of a void preferably approximates to an oval plate. The maximumdimension or length of a void (dimension "a" in FIGS. 9 and 10) isgenerally in the direction of longitudinal stretching of the film. Thewidth of a void (dimension "b" in FIG. 9) is generally in the directionof transverse stretching of the film. The depth of a void is a measureof the thickness of a void (dimension "c" in FIG. 10), ie when the filmis viewed edge on.

The mean void size or mean length of the small voids is preferably inthe range from 0.5 to 3.0 μm, more preferably 1.0 to 2.5 μm,particularly 1.3 to 2.0 μm, and especially 1.6 to 2.0 μm. The sizedistribution of the small voids is also an important parameter inobtaining a substrate exhibiting preferred characteristics. In apreferred embodiment of the invention greater than 50%, more preferablygreater than 70%, and particularly greater than 90% and up to 100% ofthe small voids have a void size or length within the range of the meanvoid size±0.3 μm, more preferably±0.2 μm, and particularly±0.1 μm.

The mean width of the small voids is preferably in the range from 0.2 to2.5 μm, more preferably 0.6 to 2.0 μm, particularly 1.0 to 1.8 μm, andespecially 1.4 to 1.6 μm.

The mean depth or thickness of the small voids is preferably in therange from 0.1 to 1.5 μm, more preferably 0.4 to 0.8 μm.

The small voids are formed around, ie contain, an inorganic fillervoiding agent which has been incorporated into the polyestersubstrate-forming composition. The inorganic filler preferably has avolume distributed median particle diameter (equivalent sphericaldiameter corresponding to 50% of the volume of all the particles, readon the cumulative distribution curve relating volume % to the diameterof the particles -- often referred to as the "D(v,0.5)" value), asdetermined by laser diffraction, of from 0.3 to 0.9 μm, more preferablyfrom 0.4 to 0.8 μm, and particularly from 0.5 to 0.7 μm.

The presence of excessively large inorganic filler particles can resultin the film exhibiting unsightly `speckle`, ie where the presence ofindividual resin particles in the film can be discerned with the nakedeye. Desirably, therefore, the actual particle size of 99.9% by volumeof the inorganic filler particles should not exceed 20 μm, andpreferably not exceed 15 μm.

Particle size of the inorganic filler particles may be measured byelectron microscope, coulter counter, sedimentation analysis and staticor dynamic light scattering. Techniques based on laser light diffractionare preferred. The median particle size may be determined by plotting acumulative distribution curve representing the percentage of particlevolume below chosen particle sizes and measuring the 50th percentile.The volume distributed median particle diameter of the filler particlesis suitably measured using a Malvern Instruments Mastersizer MS 15Particle Sizer after dispersing the filler in ethylene glycol in a highshear (eg Chemcoll) mixer.

The concentration of inorganic filler incorporated into the substrate ispreferably in the range from 14 to 19% by weight, more preferably 15 to18% by weight, and particularly 16 to 17% by weight based upon the totalweight of the components present in the substrate.

Particulate fillers suitable for generating a voided substrate includeconventional inorganic pigments and fillers, particularly metal ormetalloid oxides, such as alumina, silica and titania, and alkalinemetal salts, such as the carbonates and sulphates of calcium and barium.The inorganic filler may be homogeneous and consist essentially of asingle filler material or compound, such as titanium dioxide or bariumsulphate alone. Alternatively, at least a proportion of the filler maybe heterogeneous, the primary filler material being associated with anadditional modifying component. For example, the primary filler particlemay be treated with a surface modifier, such as a pigment, soap,surfactant coupling agent or other modifier to promote or alter thedegree to which the filler is compatible with the substrate polymer.Barium sulphate is a particularly preferred inorganic filler. In apreferred embodiment of the invention the substrate contains less than5% by weight, more preferably less than 3% by weight, particularly lessthan 1% by weight, and especially 0% by weight based upon the totalweight of the components present in the substrate, of an inorganicfiller other than barium sulphate, ie preferably barium sulphate isessentially the only inorganic filler present in the substrate.

The mean void size or mean length of the large voids is preferably inthe range from 7 to 20 μm, more preferably 9 to 19 μm, particularly 11to 18 μm, and especially 13 to 17 μm. According to the present inventionless than 15%, more preferably less than 10%, particularly less than 5%,and especially less than 3% by number of the large voids have a voidsize or length greater than 27 μm. In a particularly preferredembodiment of the invention less than 30%, more preferably less than25%, particularly less than 20%, and especially less than 15% by numberof the large voids have a void size or length greater than 21 μm.

The mean width of the large voids is preferably in the range from 5 to18 μm, more preferably 7 to 17 μm, particularly 9 to 16 μm, andespecially 11 to 15 μm.

The mean depth or thickness of the large voids is preferably in therange from 2 to 8 μm, more preferably 3 to 6 μm.

The large voids are formed around, ie contain, an organic filler voidingagent which has been incorporated into the polyester substrate-formingcomposition. A major proportion of the organic filler particles presentin the polyester substrate-forming composition, ie prior to anystretching operation, preferably have a particle size in the range from1 to 10 μm. The organic filler particles are approximately spherical,prior to film stretching, and by particle size is meant the averagediameter of a particle. Preferably greater than 70%, more preferablygreater than 80%, and particularly greater than 90% by number of theorganic filler particles have a particle size in the range from 1 to 9μm, more preferably 1 to 7 μm, and particularly 2 to 7 μm. In aparticularly preferred embodiment of the invention, suitably less than20%, preferably less than 15%, more preferably less than 10%,particularly less than 5%, and especially less than 3% by number of theorganic filler particles, prior to film stretching, have a particle sizeof greater than 9 μm. The mean particle size of the organic fillerparticles is preferably in the range from 2 to 8 μm, and more preferably3 to 6 μm.

The organic filler voiding agent is suitably an olefine polymer, such asa low or high density homopolymer, particularly polyethylene,polypropylene or poly-4-methylpentene-1, an olefine copolymer,particularly an ethylene-propylene copolymer, or a mixture of two ormore thereof. Random, block or graft copolymers may be employed.Polypropylene is a particularly preferred organic filler.

The concentration of organic filler incorporated into the substrate ispreferably in the range from 3 to 12% by weight, more preferably 4 to10% by weight, and particularly 4.5 to 7% by weight, based upon thetotal weight of the components present in the substrate.

In a preferred embodiment of the invention the ratio by number of smallvoids to large voids present in the substrate is suitably in the rangefrom 5:1 to 1000:1, preferably 25:1 to 700:1, more preferably 100:1 to600:1, particularly 150:1 to 400:1, and especially 300:1 to 400:1.

The size of the large voids is dependant, inter alia, on the size of theorganic filler particles incorporated into the polyestersubstrate-forming composition. In order to obtain filler particles ofthe preferred size, it is generally necessary to additionallyincorporate a dispersing agent together with the organic filler into thepolyester substrate-forming composition. A suitable dispersing agent,particularly for a polyolefine organic filler is a grafted polyolefinecopolymer or preferably a carboxylated polyolefine, particularly acarboxylated polyethylene.

The carboxylated polyolefine is conveniently prepared by the oxidationof an olefine homopolymer (preferably an ethylene homopolymer) tointroduce carboxyl groups onto the polyolefine chain. Alternatively thecarboxylated polyolefine may be prepared by copolymerising an olefine(preferably ethylene) with an olefinically unsaturated acid oranhydride, such as acrylic acid, maleic acid or maleic anhydride. Thecarboxylated polyolefine may, if desired, be partially neutralised.Suitable carboxylated polyolefines include those having a BrookfieldViscosity (140° C.) in the range 150-100000 cps (preferably 150-50000cps) and an Acid Number in the range 5-200 mg KOH/g (preferably 5-50 mgKOH/g), the Acid Number being the number of mg of KOH required toneutralise 1 g of polymer. The amount of dispersing agent is preferablywithin a range from 0.3 to 5.0%, more preferably 0.5 to 2.0%, andparticularly 0.8 to 1.2% by weight, relative to the weight of theorganic filler.

The inorganic filler, organic filler and/or dispersing agent may beadded to the polyester substrate or polyester substrate-forming materialat any point in the film manufacturing process prior to the extrusion ofthe polyester. For example, the inorganic filler particles may be addedduring monomer transfer or in the autoclave, although it is preferred toincorporate the particles as a glycol dispersion during theesterification reaction stage of the polyester synthesis. The inorganicfiller, organic filler and/or dispersing agent may be dry blended withthe polyester in granular or chip form prior to formation of a substratefilm therefrom, or added as a dry powder into the polyester melt via atwin-screw extruder, or by masterbatch technology. The organic filler,together with the dispersing agent, is preferably added by masterbatchtechnology.

In a preferred embodiment of the invention, the substrate comprises anoptical brightener. An optical brightener may be included at any stageof the polyester synthesis, or substrate production. It is preferred toadd the optical brightener to the glycol during polyester synthesis, oralternatively by subsequent addition to the polyester prior to theformation of the substrate, eg by injection during extrusion. Theoptical brightener is preferably added in amounts of from 50 to 1000ppm, more preferably 100 to 500 ppm, and particularly 150 to 250 ppm byweight based upon the total weight of the components present in thesubstrate. Suitable optical brighteners include those availablecommercially under the trade names "Uvitex" MES, "Uvitex" OB, "Leucopur"EGM and "Eastobrite" OB-1.

The substrate according to the invention is opaque, preferablyexhibiting a Transmission Optical Density (TOD) (Macbeth Densitometer;type TD 902; transmission mode) in the range from 1.1 to 1.45, morepreferably 1.15 to 1.4, and particularly 1.2 to 1.35, especially for a150 μm thick film.

The surface of the substrate preferably exhibits an 85° gloss value,measured as herein described, in the range from 20 to 70%, morepreferably 30 to 65%, particularly 40 to 55%, and especially 45 to 50%.

The substrate preferably exhibits a whiteness index, measured as hereindescribed, in the range from 90 to 100, more preferably 95 to 100, andparticularly 98 to 100 units.

The substrate preferably exhibits a yellowness index, measured as hereindescribed, in the range from 1 to -3, more preferably 0 to -2,particularly -0.5 to -1.5, and especially -0.8 to -1.2.

The substrate preferably exhibits a root mean square surface roughness(Rq), measured as herein described, in the range from 200 to 1500 nm,more preferably 400 to 1200 nm, and particularly 500 to 1000 nm.

The thickness of the substrate may vary depending on the envisagedapplication of the receiver sheet but, in general, will not exceed 250μm, will preferably be in a range from 50 to 190 μm, and more preferably150 to 175 μm.

When TTP is effected directly onto the surface of a substrate ashereinbefore described, the optical density of the developed image tendsto be low and it is therefore necessary to apply an additional receivinglayer to the surface of the substrate. The receiving layer desirablyexhibits (1) a high receptivity to dye thermally transferred from adonor sheet, (2) resistance to surface deformation from contact with thethermal print-head to ensure the production of an acceptably glossyprint, and (3) the ability to retain a stable image.

A receiving layer satisfying the aforementioned criteria comprises adye-receptive, synthetic thermoplastics polymer. The morphology of thereceiving layer may be varied depending on the required characteristics.For example, the receiving polymer may be of an essentially amorphousnature to enhance optical density of the transferred image, essentiallycrystalline to reduce surface deformation, or partiallyamorphous/crystalline to provide an appropriate balance ofcharacteristics.

The thickness of the receiving layer may vary over a wide range butgenerally will not exceed 50 μm. The dry thickness of the receivinglayer governs, inter alia, the optical density of the resultant imagedeveloped in a particular receiving polymer, and preferably is within arange of from 0.5 to 25 μm. In particular, it has been observed that bycareful control of the receiving layer thickness to within a range offrom 0.5 to 10 μm, in association with an opaque substrate layer of thekind herein described, a surprising and significant improvement inresistance to surface deformation is achieved, without significantlydetracting from the optical density of the transferred image.

A dye-receptive polymer for use in the receiving layer suitablycomprises a polyester resin, a polyvinyl chloride resin, or copolymersthereof such as a vinyl chloride/vinyl alcohol copolymer.

A suitable copolyester resin derived from one or more dibasic aromaticcarboxylic acids, such as terephthalic acid, isophthalic acid andhexahydroterephthalic acid, and one or more glycols, such as ethyleneglycol, diethylene glycol, triethylene glycol and neopentyl glycol.Typical copolyesters which provide satisfactory dye-receptivity anddeformation resistance are those of ethylene terephthalate and ethyleneisophthalate, particularly in the molar ratios of from 50 to 90 mole %ethylene terephthalate and correspondingly from 10 to 50 mole % ethyleneisophthalate. Preferred copolyesters comprise from 65 to 85 mole %ethylene terephthalate and from 15 to 35 mole % ethylene isophthalate. Aparticularly preferred copolyester comprises approximately 82 mole %ethylene terephthalate and 18 mole % ethylene isophthalate.

Preferred commercially available amorphous polyesters include "VitelPE200" (Goodyear) and "Vylon" polyester grades 103, 200 and 290(Toyobo). Mixtures of different polyesters may be present in thereceiving layer.

Formation of a receiving layer on the receiver sheet may be effected byconventional techniques, for example by casting the polymer onto apreformed substrate, followed by drying at an elevated temperature.Drying of a receiver sheet comprising a polyester substrate and acopolyester receiving layer is conveniently effected at a temperaturewithin a range of from 175 to 250° C. Conveniently, however, formationof a composite sheet (substrate and receiving layer) is effected bycoextrusion, either by simultaneous coextrusion of the respectivefilm-forming layers through independent orifices of a multi-orifice die,and thereafter uniting the still molten layers, or, preferably, bysingle-channel coextrusion in which molten streams of the respectivepolymers are first united within a channel leading to a die manifold,and thereafter extruded together from the die orifice under conditionsof streamline flow without intermixing thereby to produce a compositesheet.

A coextruded sheet is stretched to effect molecular orientation of thesubstrate, and preferably heat-set, as hereinbefore described.Generally, the conditions applied for stretching the substrate layerwill induce partial crystallisation of the receiving polymer and it istherefore preferred to heat set under dimensional restraint at atemperature selected to develop the desired morphology of the receivinglayer. Thus, by effecting heat-setting at a temperature below thecrystalline melting temperature of the receiving polymer and permittingor causing the composite to cool, the receiving polymer will remainessentially crystalline. However, by heat-setting at a temperaturegreater than the crystalline melting temperature of the receivingpolymer, the latter will be rendered essentially amorphous. Heat-settingof a receiver sheet comprising a polyester substrate and a copolyesterreceiving layer is conveniently effected at a temperature within a rangeof from 175 to 200° C. to yield a substantially crystalline receivinglayer, or from 200 to 250° C. to yield an essentially amorphousreceiving layer.

In one embodiment of the invention, an adherent layer is present betweenthe substrate and receiving layer. The function of the additionaladherent layer is to increase the strength of adhesion of the receivinglayer to the substrate. The adherent layer preferably comprises anacrylic resin, by which is meant a resin comprising at least one acrylicand/or methacrylic component.

The acrylic resin component of the adherent layer is preferablythermoset, and preferably comprises at least one monomer derived from anester of acrylic acid and/or an ester of methacrylic acid, and/orderivatives thereof. In a preferred embodiment of the invention, theacrylic resin comprises from 50 to 100 mole %, more preferably 70 to 100mole %, particularly 80 to 100 mole %, and especially 85 to 98 mole % ofat least one monomer derived from an ester of acrylic acid and/or anester of methacrylic acid, and/or derivatives thereof. A preferredacrylic resin for use in the present invention preferably comprises analkyl ester of acrylic and/or methacrylic acid where the alkyl groupcontains up to ten carbon atoms such as methyl, ethyl, n-propyl,isopropyl, n-butyl, isobutyl, terbutyl, hexyl, 2-ethylhexyl, heptyl, andn-octyl. Polymers derived from an alkyl acrylate, for example ethylacrylate and/or butyl acrylate, together with an alkyl methacrylate arepreferred. Polymers comprising ethyl acrylate and methyl methacrylateare particularly preferred. The acrylate monomer is preferably presentin the acrylic resin in a proportion in the range from 30 to 65 mole %,and the methacrylate monomer is preferably present in a proportion inthe range from 20 to 60 mole %.

Other monomers which are suitable for use in the preparation of thepreferred acrylic resin of the adherent layer, which may be preferablycopolymerised as optional additional monomers together with esters ofacrylic acid and/or methacrylic acid, and/or derivatives thereof,include acrylonitrile, methacrylonitrile, halo-substitutedacrylonitrile, halo-substituted methacrylonitrile, acrylamide,methacrylamide, N-methylol acrylamide, N-ethanol acrylamide, N-propanolacrylamide, N-methacrylamide, N-ethanol methacrylamide, N-methylacrylamide, N-tertiary butyl acrylamide, hydroxyethyl methacrylate,glycidyl acrylate, glycidyl methacrylate, dimethylamino ethylmethacrylate, itaconic acid, itaconic anhydride and half esters ofitaconic acid.

Other optional monomers of the acrylic resin adherent layer polymerinclude vinyl esters such as vinyl acetate, vinyl chloroacetate andvinyl benzoate, vinyl pyridine, vinyl chloride, vinylidene chloride,maleic acid, maleic anhydride, styrene and derivatives of styrene suchas chloro styrene, hydroxy styrene and alkylated styrenes, wherein thealkyl group contains from one to ten carbon atoms.

A preferred acrylic resin, derived from 3 monomers comprises 35 to 60mole % of ethyl acrylate/30 to 55 mole % of methyl methacrylate/2 to 20mole % of acrylamide or methacrylamide, and particularly comprisingapproximate molar proportions 46/46/8 mole % respectively of ethylacrylate/methyl methacrylate/acrylamide or methacrylamide, the latterpolymer being especially effective when thermoset, for example in thepresence of about 25 weight % of a methylated melamine formaldehyderesin.

A preferred acrylic resin, derived from 4 monomers comprises a copolymercomprising comonomers (a) 35 to 40 mole % alkyl acrylate, (b) 35 to 40mole % alkyl methacrylate, (c) 10 to 15 mole % of a monomer containing afree carboxyl group and/or a salt thereof, and (d) 15 to 20 mole % of asulphonic acid and/or a salt thereof. Ethyl acrylate is a particularlypreferred monomer (a), and methyl methacrylate is a particularlypreferred monomer (b). Monomer (c) containing a free carboxyl groupand/or a salt thereof, ie a carboxyl group other than those involved inany polymerisation reaction by which the copolymer may be formed,suitably comprises a copolymerisable unsaturated carboxylic acid, and ispreferably selected from acrylic acid, methacrylic acid, maleic acid,and/or itaconic acid. Acrylic acid and itaconic acid are particularlypreferred. The sulphonic acid monomer (d) may also be present as thefree acid and/or a salt thereof. Preferred salts include the ammonium,substituted ammonium, or an alkali metal, such as lithium, sodium orpotassium, salt. The sulphonate group does not participate in thepolymerisation reaction by which the adherent copolymer resin is formed.The sulphonic acid monomer preferably contains an aromatic group, andmore preferably is p-styrene sulphonic acid and/or a salt thereof.

The weight average molecular weight of the acrylic resin can vary over awide range but is preferably within the range 10,000 to 10,000,000, andmore preferably within the range 50,000 to 200,000.

The acrylic resin preferably comprises at least 30%, more preferably inthe range from 40% to 95%, particularly 60% to 90%, and especially 70%to 85% by weight, relative to the total weight of the dry adherentlayer. The acrylic resin is generally water-insoluble. The coatingcomposition including the water-insoluble acrylic resin may neverthelessbe applied to the substrate as an aqueous dispersion. A suitablesurfactant may be included in the coating composition in order to aidthe dispersion of the acrylic resin.

If desired, the adherent layer coating composition may also contain across-linking agent which functions to cross-link the layer therebyimproving adhesion to the substrate. Additionally, the cross-linkingagent should preferably be capable of internal cross-linking in order toprovide protection against solvent penetration. Suitable cross-linkingagents may comprise epoxy resins, alkyd resins, amine derivatives suchas hexamethoxymethyl melamine, and/or condensation products of an amine,eg melamine, diazine, urea, cyclic ethylene urea, cyclic propylene urea,thiourea, cyclic ethylene thiourea, alkyl melamines, aryl melamines,benzo guanamines, guanamines, alkyl guanamines and aryl guanamines, withan aldehyde, eg formaldehyde. A useful condensation product is that ofmelamine with formaldehyde. The condensation product may optionally bealkoxylated. The cross-linking agent may suitably be used in amounts inthe range from 5% to 60%, preferably 10% to 40%, more preferably 15% to30% by weight, relative to the total weight of the dry adherent layer. Acatalyst is also preferably employed to facilitate cross-linking actionof the cross-linking agent. Preferred catalysts for cross-linkingmelamine formaldehyde include para toluene sulphonic acid, maleic acidstabilised by reaction with a base, morpholinium paratoluene sulphonate,and ammonium nitrate.

The adherent layer coating composition may be applied before, during orafter the stretching operation in the production of an oriented film.The adherent layer coating composition is preferably applied to thesubstrate between the two stages (longitudinal and transverse) of athermoplastics polyester film biaxial stretching operation. Such asequence of stretching and coating is suitable for the production of anadherent layer coated linear polyester film, particularly a polyethyleneterephthalate film substrate, which is preferably firstly stretched inthe longitudinal direction over a series of rotating rollers, coated,and then stretched transversely in a stenter oven, preferably followedby heat setting.

The adherent layer coating composition is preferably applied to thesubstrate by any suitable conventional technique such as dip coating,bead coating, reverse roller coating or slot coating.

The adherent layer is preferably applied to the substrate at a coatweight within the range from 0.05 to 10 mgdm⁻², and more preferably 0.1to 2.0 mgdm⁻². For a substrate coated on both surfaces, each adherentlayer preferably has a coat weight within the preferred range.

Prior to deposition of the adherent layer onto the substrate, theexposed surface thereof may, if desired, be subjected to a chemical orphysical surface-modifying treatment to improve the bond between thatsurface and the subsequently applied adherent layer. A preferredtreatment, because of its simplicity and effectiveness, is to subjectthe exposed surface of the substrate to a high voltage electrical stressaccompanied by corona discharge.

If desired, a receiver sheet according to the invention may additionallycomprise an antistatic layer. Such an antistatic layer is convenientlyprovided on a surface of the substrate remote from the receiving layer.Although a conventional antistatic agent may be employed, a polymericantistat is preferred. A particularly suitable polymeric antistat isthat described in EP-A-0349152, the disclosure of which is incorporatedherein by reference, the antistat comprising (a) a polychlorohydrinether of an ethoxylated hydroxyamine and (b) a polyglycol diamine, thetotal alkali metal content of components (a) and (b) not exceeding 0.5%of the combined weight of (a) and (b).

A receiver sheet in accordance with the invention may, if desired,comprise a release medium present either within the receiving layer or,preferably as a discrete layer on at least part of the exposed surfaceof the receiving layer remote from the substrate.

The release medium, if employed, should be permeable to the dyetransferred from the donor sheet, and comprises a release agent, forexample of the kind conventionally employed in TTP processes to enhancethe release characteristics of a receiver sheet relative to a donorsheet. Suitable release agents include solid waxes, fluorinatedpolymers, silicone oils (preferably cured) such as epoxy- and/oramino-modified silicone oils, and especially organopolysiloxane resins.A particularly suitable release medium comprises a polyurethane resincomprising a poly dialkylsiloxane as described in EP-A-0349141, thedisclosure of which is incorporated herein by reference.

The invention is illustrated by reference to the accompanying drawingsin which:

FIG. 1 is a schematic elevation (not to scale) of a portion of a TTPreceiver sheet (1) comprising a supporting substrate (2) having, on afirst surface thereof, a dye-receptive receiving layer (3).

FIG. 2 is a similar, fragmentary schematic elevation in which thereceiver sheet comprises an additional adherent layer (4).

FIG. 3 is a schematic, fragmentary elevation (not to scale) of acompatible TTP donor sheet (5) comprising a substrate (6) having on onesurface (the front surface) thereof a transfer layer (7) comprising asublimable dye in a resin binder, and on a second surface (the rearsurface) thereof a polymeric protective layer (8).

FIG. 4 is a schematic elevation of a TTP process employing the receiversheet shown in FIG. 2 and the donor sheet shown in FIG. 3, and

FIG. 5 is a schematic elevation of an imaged receiver sheet.

FIG. 6 is a sectional plan view (not to scale) of a portion of anundrawn substrate (precursor substrate of receiver sheet) comprising apolyester matrix (12) having dispersed therein both organic fillerparticles (13) and inorganic filler particles (14).

FIG. 7 is a similar sectional plan view of a biaxially orientedsubstrate of the receiver sheet illustrating the voids (15) and (16)formed around the organic filler particles (13) and inorganic fillerparticles (14) respectively.

FIG. 8 is a sectional elevation, ie an edge on view, of the orientedsubstrate shown in FIG. 7, providing an alternative view of the voids(15) and (16) formed around the organic filler particles (13) andinorganic filler particles (14) respectively.

FIG. 9 is a sectional plan view of an individual large void present inthe film shown in FIG. 7, illustrating the size or length (dimension"a") and width (dimension "b") of a void.

FIG. 10 is a sectional elevation of an individual large void present inthe film shown in FIG. 8, illustrating the size or length (dimension"a") and depth or thickness (dimension "c") of a void.

Referring to FIGS. 4 and 5 of the drawings, a TTP process is effected byassembling a donor sheet and a receiver sheet with the respectivetransfer layer (7) and receiving layer (4) in contact. Anelectrically-activated thermal print-head (9) comprising a plurality ofprint elements (only one of which is shown (10)) is then placed incontact with the protective layer of the donor sheet. Energisation ofthe print-head causes selected individual print-elements (10) to becomehot, thereby causing dye from the underlying region of the transferlayer to sublime into receiving layer (4) where it forms an image (11)of the heated element(s). The resultant imaged receiver sheet, separatedfrom the donor sheet, is illustrated in FIG. 5 of the drawings.

By advancing the donor sheet relative to the receiver sheet, andrepeating the process, a multi-colour image of the desired form may begenerated in the receiving layer.

In this specification the following test methods have been used todetermine certain properties of the substrate and receiver sheet:

(i) Transmission Optical Density (TOD)

TOD of the film was measured using a Macbeth Densitometer TD 902(obtained from Dent and Woods Ltd. Basingstoke, UK) in transmissionmode.

(ii) Gloss Value

The 85° gloss value of the film surface was measured using a Dr LangeReflectometer RB3 (obtained from Dr Bruno Lange, GmbH, Dusseldorf,Germany) based on the principles described in ASTM D 523.

(iii) Whiteness Index and Yellowness Index

The whiteness index and yellowness index of the film was measured usinga Colorgard System 2000, Model/45 (manufactured by Pacific Scientific)based on the principles described in ASTM D 313.

(iv) Surface Roughness

The film surface root mean square roughness (Rq) was measured using aRank Taylor-Hobson Talysurf 10 (Leicester, UK) employing a cut-offlength of 0.25 mm.

(v) Void Size

The size of the voids was determined by fracturing, after freezing innitrogen, a sample of the substrate of the receiver sheet, followed bysputtering with gold. Scanning electron microscope micrographs wereprepared, and measurements taken of at least 100, more preferably atleast 500, and particularly at least 1000 small voids and large voids.Mean void size or mean length of the small voids and large voids wascalculated. In addition, the % of large voids having a void size orlength greater than 21 μm, and greater than 27 μm was determined. Themeasurement of the void size can be performed by eye or by ImageAnalysis, for example using a Kontron IBAS system.

The invention is further illustrated by reference to the followingExamples.

EXAMPLE 1

A substrate layer composition comprising the following ingredients:

    ______________________________________    Polyethylene terephthalate                              74 wt %    Polypropylene             9.6 wt %    Carboxylated polyethylene 0.1 wt %    ("AC" wax, supplied by Allied Chemicals)    Barium sulphate           16.3 wt %    (volume distributed median particle    diameter = 0.6 μm)    ______________________________________

was prepared by first compounding the carboxylated polyethylene into thepolypropylene, and using as a masterbatch. The substrate composition wasmelt extruded, cast onto a cooled rotating drum and stretched in thedirection of extrusion to approximately 3.1 times its originaldimensions. The film passed into a stenter oven, where the film wasstretched in the sideways direction to approximately 3.3 times itsoriginal dimensions. The biaxially stretched film was heat set at atemperature of about 220° C. by conventional means. Final film thicknesswas 175 μm.

The substrate film was subjected to the test procedures described hereinand exhibited the following properties.

(i) Transmission Optical Density (TOD)=1.35

(ii) 85° gloss value=31%

(iii) Whiteness Index=99.3 units

Yellowness Index=-1.1 units

(iv) Root mean square roughness (Rq)=800 nm (v) Mean void size of thesmall voids=1.8 μm

Mean void size of the large voids=15.3 μm

Number of large voids having a void size>21 μm=18%

Number of large voids having a void size>27 μm=3%

A polyester receiving layer was coated directly onto the surface of thesubstrate.

The printing characteristics of the film were assessed using a donorsheet comprising a biaxially oriented polyethylene terephthalatesubstrate of about 6 μm thickness having on one surface thereof atransfer layer of about 2 μm thickness comprising a magenta dye in acellulosic resin binder.

A sandwich comprising a sample of the donor and receiver sheets with therespective transfer and receiving layers in contact was placed on therubber covered drum of a thermal transfer printing machine and contactedwith a print head comprising a linear array of pixels spaced apart at alinear density of 6/mm. On selectively heating the pixels in accordancewith a pattern information signal to a temperature of about 350° C.(power supply 0.32 watt/pixel) for a period of 10 milliseconds (ms),magenta dye was transferred from the transfer layer of the donor sheetto form a corresponding image of the heated pixels in the receivinglayer of the receiver sheet.

After stripping the transfer sheet from the coated film, the band imageon the latter was assessed visually, and no printing flaws (unprintedspots or areas of relatively low optical density) were observed.

EXAMPLE 2

The substrate produced in Example 1 was additionally coated with anadherent layer, prior to applying the polyester receiving layer, ie thereceiving layer was applied to the surface of the adherent layer. Theadherent layer coating composition was applied to the monoaxiallyoriented polyethylene terephthalate substrate, ie prior to the sidewaysstretching. The adherent layer coating composition comprised thefollowing ingredients:

    ______________________________________    Acrylic resin            163 ml    (46% w/w aqueous latex of methyl    methacrylate/ethyl acrylate/methacrylamide:    46/46/8 mole %, with 25% by weight    methoxylated melamine-formaldehyde)    Ammonium nitrate         12.5 ml    (10% w/w aqueous solution)    Synperonic NDB           30 ml    (13.7% w/w aqueous solution of a nonyl phenol    ethoxylate, supplied by ICI)    Demineralised water      to 2.5 liters    ______________________________________

The adherent layer coated film was passed into a stenter oven, where thefilm was stretched in the sideways direction and heat-set as describedin Example 1. The dry coat weight of the adherent layer wasapproximately 0.4 mgdm⁻² and the thickness of the adherent layer wasapproximately 0.04 μm. The polyester receiving layer described inExample 1 was coated directly on to the surface of the acrylic adherentlayer to form the receiver sheet.

The printing characteristics of the receiver sheet were evaluated usingthe test procedures described in Example 1, and again no printing flawswere observed.

EXAMPLE 3

The procedure of Example 2 was repeated except that substrate layercomposition comprised the following ingredients:

    ______________________________________    Polyethylene terephthalate                              78 wt %    Polypropylene             5 wt %    Carboxylated polyethylene 0.05 wt %    ("AC" wax, supplied by Allied Chemicals)    Barium sulphate           17 wt %    (volume distributed median particle    diameter = 0.6 μm)    ______________________________________

The substrate film was subjected to the test procedures described hereinand exhibited the following properties.

(i) Transmission Optical Density (TOD)=1.26

(ii) 85° gloss value=46%

(iii) Whiteness Index=98 units

Yellowness Index=-1 units

(iv) Root mean square roughness (Rq)=600 nm

(v) Mean void size of the small voids=1.75 μm

Mean void size of the large voids=15 μm

Number of large voids having a void size>21 μm=15%

Number of large voids having a void size>27 μm=2%

The polyester receiving layer described in Example 1 was coated directlyonto the surface of the acrylic adherent layer to form the receiversheet.

The printing characteristics of the receiver sheet were evaluated usingthe test procedures described in Example 1, and again no printing flawswere observed.

EXAMPLE 4

This is a comparative example not according to the invention. Theprocedure of Example 2 was repeated except that substrate layercomposition comprised 0.05 wt % of carboxylated polyethylene.

The substrate film exhibited the following void characteristics.

(i) Mean void size of the small voids=1.8 μm

Mean void size of the large voids=16 μm

Number of large voids having a void size>27 μm=18%

The polyester receiving layer described in Example 1 was coated directlyonto the surface of the acrylic adherent layer to form the receiversheet.

The printing characteristics of the receiver sheet were evaluated usingthe test procedures described in Example 1, and printing flaws wereobserved.

The above examples illustrate the improved properties of a receiversheet according to the present invention.

We claim:
 1. A thermal transfer printing receiver sheet for use inassociation with a compatible donor sheet, the receiver sheet comprisinga dye-receptive receiving layer to receive a dye thermally transferredfrom the donor sheet, and an opaque biaxially oriented supportingpolyester substrate comprising (i) small voids, formed around inorganicfiller particles, having a mean void size in the range from 0.3 to 3.5μm, and (ii) large voids, formed around organic filler particles, havinga mean void size in the range from 5 to 21 μm and less than 15% bynumber of the large voids have a void size greater than 27 μm.
 2. Areceiver sheet according to claim 1 wherein less than 10% by number ofthe large voids have a void size greater than 27 μm.
 3. A receiver sheetaccording to claim 2 wherein less than 5% by number of the large voidshave a void size greater than 27 μm.
 4. A receiver sheet according toclaim 1 wherein less than 30% by number of the large voids have a voidsize greater than 21 μm.
 5. A receiver sheet according to claim 4wherein less than 20% by number of the large voids have a void sizegreater than 21 μm.
 6. A receiver sheet according to claim 1 wherein theconcentration of organic filler particles in the substrate is in therange from 3 to 12% by weight, based upon the total weight of thecomponents present in the substrate.
 7. A receiver sheet according toclaim 1 wherein the concentration of inorganic filler particles in thesubstrate is in the range from 14 to 19% by weight, based upon the totalweight of the components present in the substrate.
 8. A receiver sheetaccording to claim 1 wherein the ratio by number of small voids to largevoids in the substrate is in the range from 25:1 to 700:1.
 9. A receiversheet according to claim 1 wherein the substrate has a root mean squaresurface roughness (Rq) in the range from 400 to 1200 nm.
 10. A method ofproducing a thermal transfer printing receiver sheet for use inassociation with a compatible donor sheet, which comprises forming anopaque biaxially oriented supporting polyester substrate comprising (i)small voids, formed around inorganic filler particles, having a meanvoid size in the range from 0.3 to 3.5 μm, and (ii) large voids, formedaround organic filler particles, having a mean void size in the rangefrom 5 to 21 μm and less than 15% by number of the large voids have avoid size greater than 27 μm, and applying on at least one surface ofthe substrate, a dye-receptive receiving layer to receive a dyethermally transferred from the donor sheet.